The removal of lead based paint from bridges and highway overpasses is now mandated by federal and state authorities. Sand blasting is the most efficient and commonly used method of removing this paint in order to prepare the bridges for repainting with a nontoxic coating. For example, U.S. Pat. No. 2,869,291 to Clay discloses a mobile shot blasting machine that may be used to remove paint from a roadway bridge.
Sand blasting, however, sends paints chips flying and leaves a large residue of sand grains and leaded dust. This debris should be contained and collected to the greatest extent possible. According to the March, 1993 issue of Mechanical Engineering, sandblasting a single medium sized highway bridge will typically generate 800 tons of grit having an untreated disposal cost of $400,000. Effectively containing the debris cuts down on the disposal cost, as scattered debris does not need to be collected. Separating the paint waste and lead dust from the used blasting sand so that the sand can be recycled also cuts down on the cost of the overall operation.
U.K. Patent Application GB 2 193 454 A to Auto Truck Cleaning Ltd. discloses a mobile shot blast cleaning apparatus that collects all used blasting shot and paint residue. The apparatus has a special nozzle that is connected to a flexible conduit which supplies pressurized shot to the nozzle, and to an outlet pipe that draws used shot and paint debris from the nozzle. While this apparatus is efficient at collecting the waste materials, it is not suitable for use in removing paint from a highway bridge as the nozzle only allows processing of a small area. Further, ladders or scaffolding would be necessary in order to reach the bridge with the nozzle.
In order to contain the debris as it is blasted from the bridge, a heavy curtain is usually hung around the work area, reaching to the ground below. This curtain encircles the work area of the bridge on all sides during the sand blasting procedure. For example, U.S. Pat. No. 4,852,307 to Goudeau discloses a shroud that is placed over the object to be sandblasted, along with inflatable collection baskets for retrieving sandblasted cuttings.
In order to collect small lead dust particles present in the air during blasting, air inlets and outlets connected to fans are usually employed to continually replace the air in the curtained work area. Because the work area is a large enclosed space spanning a section of the bridge from bridge level to the ground below, many high powered fans are necessary to keep the air recirculating sufficiently. The number and power of such fans is directly related to the volume of work space enclosed by the curtains. Providing and operating these fans is a substantial contribution to the overall expense incurred in a sand blasting operation.
For example, U.S. Pat. No. 4,787,179 to Lewis discloses an abrasive blasting containment system having a complex air circulation system. Air is pumped into an enclosed room through an inlet having diffusers which minimize air turbulence. Air return ducts are utilized to draw air out of the room. Items to be sandblasted are brought into the room where a worker applies a sand blasting apparatus.
Because a highway bridge cannot be brought into a room to be sand blasted, erection of a curtain and sand blasting of a bridge currently requires the use of scaffolding or similar structures. The scaffolding is placed on the ground below the bridge, providing support for the curtain and allowing workers to reach the bridge. This scaffolding must sometimes be placed directly on the roadway running beneath the bridge or overpass. Because this can cause traffic congestion on the roadway, sand blasting often takes place at night.
Setting up the scaffolding, curtain, and blasting equipment takes a considerable amount of time and manpower using conventional systems. An alternative to scaffolding is disclosed in U.S. Pat. No. 5,011,710 to Harrison. This patent discloses a walkway within an enclosure that can be suspended from a structure. Workers on the walkway sand blast the structure, and a vacuum conveyor collects and removes particles resulting from the blasting procedure. This apparatus has less effect on traffic than scaffolding does, but requires a great deal of time to set up and mount on a bridge. Breakdown of the apparatus is time consuming as well.
Moving the scaffolding setup as work progresses is also a time consuming operation. Local authorities often limit the night time hours that a sand blasting crew is allowed to work so that rush hour traffic is not impeded. Under these restrictions, it is often the case that set up and break down times for the operation are significantly longer than the time that sand blasting actually takes place. Part of this time involves cleanup and removal of toxic lead chips and dust from the roadway and surrounding area. Thus, a sand blasting job using conventional equipment is an inefficient task. Decreasing environmental pollution from said blasting jobs is a key goal. Further, decreasing the time it takes to set up and break down the sand blasting apparatus would dramatically increase the productivity of such a job thereby allowing more lead based paint to be removed in a given amount of time.